1. Field of the Invention
The present invention relates to the production and use of modified carbon products in fuel cell components and similar devices. Specifically, the present invention relates to electrodes and electrocatalyst layers incorporating modified carbon products and methods for making electrodes and electrocatalyst layers including modified carbon products. The modified carbon products can be used to enhance and tailor the properties of the electrode and electrocatalyst layers.
2. Description of Related Art
Fuel cells are electrochemical devices that are capable of converting the energy of a chemical reaction into electrical energy without combustion and with virtually no pollution. Fuel cells are unlike batteries in that fuel cells convert chemical energy to electrical energy as the chemical reactants are continuously delivered to the fuel cell. As a result, fuel cells are used to produce a continuous source of electrical energy, and compete with other forms of continuous energy production such as the combustion engine, nuclear power and coal-fired power stations. Different types of fuel cells are categorized by the electrolyte used in the fuel cell. The five main types of fuel cells are alkaline, molten carbonate, phosphoric acid, solid oxide and proton exchange membrane (PEM), also known as polymer electrolyte fuel cells (PEFCs). One particularly useful fuel cell is the proton exchange membrane fuel cell (PEMFC).
A PEMFC typically includes tens to hundreds of MEAs each of which includes a cathode layer and an anode layer. One embodiment of a MEA is illustrated in FIGS. 1(a) and 1(b). One embodiment of a cathode side of an MEA is also depicted in FIG. 2. With references to FIGS. 1(a), 1(b) and 2, the anode electrocatalyst layer 104 and cathode electrocatalyst layer 106 sandwich a proton exchange membrane 102. In some instances, the combined membrane and electrode layer is referred to as a catalyst coated membrane 103. Power is generated when a fuel (e.g., hydrogen gas) is fed into the anode 104 and oxygen (air) 106 is fed into the cathode. In a reaction typically catalyzed by a platinum-based catalyst in the catalyst layer of the anode 104, the hydrogen ionizes to form protons and electrons. The protons are transported through the proton exchange membrane 102 to a catalyst layer on the opposite side of the membrane (the cathode), where another catalyst, typically platinum or a platinum alloy, catalyzes an oxygen-reduction reaction to form water. The reactions can be written as follows:Anode: 2H2→4H++4e−  (1)Cathode: 4H++4e−+O2→2H2O  (2)Overall: 2H2+O2→2H2O  (3)
Electrons formed at the anode and cathode are routed through bipolar plates 114 connected to an electrical circuit. On either side of the anode 104 and cathode 106 are porous gas diffusion layers 108, which generally comprise a carbon support layer 107 and a microporous layer 109, that help enable the transport of reactants (H2 and O2 when hydrogen gas is the fuel) to the anode and the cathode. On the anode side, fuel flow channels 110 may be provided for the transport of fuel, while on the cathode side, oxidizer flow channels 112 may be provided for the transport of an oxidant. These channels may be located in the bipolar plates 114. Finally, cooling water passages 116 can be provided adjacent to or integral with the bipolar plates for cooling the MEA/fuel cell.
A particularly preferred fuel cell for portable applications, due to its compact construction, power density, efficiency and operating temperature, is a PEMFC that can utilize methanol (CH3OH) directly without the use of a fuel reformer to convert the methanol to H2. This type of fuel cell is typically referred to as a direct methanol fuel cell (DMFC). DMFCs are attractive for applications that require relatively low power, because the anode reforms the methanol directly into hydrogen ions that can be delivered to the cathode through the PEM. Other liquid fuels that may also be used in a fuel cell include formic acid, formaldehyde, ethanol and ethylene glycol.
Like a PEMFC, a DMFC also is made of a plurality of membrane electrode assemblies (MEAs). A cross-sectional view of a typical MEA is illustrated in FIG. 3 (not to scale). The MEA 300 comprises a PEM 302, an anode electrocatalyst layer 304, cathode electrocatalyst layer 306, fluid distribution layers 308, and bipolar plates 314. The electrocatalyst layers 304, 306 sandwich the PEM 302 and catalyze the reactions that generate the protons and electrons to power the fuel cell, as shown below. The fluid diffusion layer 308 distributes the reactants and products to and from the electrocatalyst layers 304, 306. The bipolar plates 314 are disposed between the anode and cathode of sequential MEA stacks, and comprise current collectors 317 and fuel and oxidizer flow channels, 310, 312, respectively, for directing the flow of incoming reactant fluid to the appropriate electrode. Two end plates (not shown), similar to the bipolar plates, are used to complete the fuel cell stack.
Operation of the DMFC is similar to a hydrogen-gas based PEMFC, except that methanol is supplied to the anode instead of hydrogen gas. Methanol flows through the fuel flow channels 310 of bipolar plate 314, through the fluid distribution layer 308 and to the anode electrocatalyst layer 304, where it decomposes into carbon dioxide gas, protons and electrons. Oxygen flows through the oxidizer flow channels 312 of the bipolar plate 314, through the fluid distribution layer 308, and to the cathode electrocatalyst layer, where ionized oxygen is produced. Protons from the anode pass through the PEM 302, and recombine with the electrons and ionized oxygen to form water. Carbon dioxide is produced at the anode 304 and is removed through the exhaust of the cell. The foregoing reactions can be written as follows:Anode: CH3OH+H2O→CO2+6H++6e−  (4)Cathode: 6H++6e−+ 3/2O2→3H2O  (5)Overall: 2CH3OH+3O2→2CO2+6H2O+energy  (6)
There are a number of properties that are required for efficient fuel cell operation. Typical properties desired for electrode operation include high utilization of catalysts and effective mass transport. Typically, electrocatalyst materials, such as platinum (Pt) dispersed on a conductive support (e.g. carbon), are utilized in the electrode layers to catalyze the foregoing reactions. Current PEMFCs typically include platinum electrocatalysts with a total loading of about 1 milligram of platinum per square centimeter of electrode (1 mgPt/cm2), including both the anode and cathode. At a typical cell performance of 0.42 Watts per square centimeter, about 2.4 grams of platinum per kilowatt is required (1 mgPt/cm2 over 0.42 Watts/cm2). In the case of liquid fuel, such as methanol, alloy catalyst such as PtRu can be used. Current DMFCs typically include PtRu electrocatalysts with a total loading of about 8 mg (Pt+Ru)/cm2, which equates to about 200 grams of precious metal per kilowatt at a performance of 0.04 Watts per square centimeter.
Platinum metal is very expensive, which often equates to expensive fuel cell production costs. Reducing the amount of catalytically active material in the electrode, however, is not a suitable solution to reduce cost, because of the equivalently strong demand for improved cell performance. Moreover, excess platinum metal is often used to compensate for catalyst materials that go unutilized or are underutilized due to their remote location from gas/fluid distribution channels and/or proton conduction sites. Thus, there is a need to enhance catalyst use in the electrode to maintain performance, but at lower loading levels.
Other issues associated with increasing the performance of the electrodes include: a) prevention of active phase agglomeration and loss of catalyst active area during the operation of the fuel cell; b) locating the proton conductive sites proximal to the active catalytic sites; c) ensuring long term stability of the electrode performance by minimizing the amount of impurities that poison the membrane and active sites; and d) enhancing porosity of the electrode for transport of reactants and products.
Carbon is a material that has previously been used for some components of the fuel cell structure. For example, U.S. Pat. No. 6,280,871 by Tosco et al. discloses gas diffusion electrodes containing carbon products. The carbon product can be used for at least one component of the electrodes, such as the active layer and/or the blocking layer. Methods to extend the service life of the electrodes, as well as methods to reduce the amount of fluorine-containing compounds are also disclosed. Similar products and methods are described in U.S. Pat. No. 6,399,202 by Yu et al. Each of the foregoing patents is incorporated herein by reference in its entirety.
U.S. patent application Publication No. 2003/0017379 by Menashi, which is incorporated herein by reference in its entirety, discloses fuel cells including a gas diffusion electrode, gas diffusion counter-electrode, and an electrolyte membrane located between the electrode and counter-electrode. The electrode, counter-electrode, or both, contain at least one carbon product. The electrolyte membranes can also contain carbon products. Similar products and methods are described in U.S. patent application Publication No. 2003/0022055 by Menashi, which is also incorporated herein by reference in its entirety.
U.S. patent application Publication No. 2003/0124414 by Hertel et al., which is incorporated herein by reference in its entirety, discloses a porous carbon body for a fuel cell having an electronically conductive hydrophilic agent and discloses a method for the manufacture of the carbon body. The porous carbon body comprises an electronically conductive graphite powder in an amount of between 60 and 80 weight percent of the body, carbon fiber in an amount of between 5 and 15 weight percent of the body, a thermoset binder in an amount between 6 and 18 weight percent of the body and an electronically created modified carbon black. Hertel et al. disclose that the carbon body provides increased wettability without any decrease in electrical conductivity, and can be manufactured without high temperature steps to add graphite to the body or to incorporate post molding hydrophilic agents into pores of the body.